1. Technical Field
The present invention relates generally to an occupant protection system for automotive vehicles having a plurality of occupant protection units such as airbags and pretensioners for seat belts, and more particularly to an improvement of a control circuit designed to activate at least one of occupant protection units in the event of a collision, even if a failure of power supplied to the control circuit from a vehicle's storage battery occurs.
2. Background of Related Art
FIG. 3 shows a conventional control circuit for an occupant protection system for an automotive vehicle having mounted therein a pair of airbags.
The control circuit includes a single backup capacitor and two safing sensors 2 and 3. When an acceleration sensor 4 detects sudden deceleration of the vehicle caused by a vehicle collision, a microcomputer 5 turns on both transistors 6 and 7, thereby supplying the voltage of a vehicle's storage battery 15 to a squib 8 through an ignition switch 17, a diode 1a, and the safing sensor 2 and also to a squib 9 through the ignition switch 17 and the safing sensor 3 for inflating the pair of airbags.
If a connection between the battery 15 and the control circuit is broken due to a collision of the vehicle, a microcomputer 5 turns on the transistor 6 in response to a signal from the acceleration sensor 4 and applies the voltage from the backup capacitor 1 to the squib 8 through the safing sensor 2 for activating one of the airbags.
The above conventional control circuit, however, encounters the drawback in that the use of the safing sensors equal to the number of airbags results in an increase in the number of component parts of the control circuit, thus leading to increased total costs of the system.
Additionally, since the control circuit provides the voltage of the backup capacitor 1 only to the squib 8, it is impossible to activate the squib 9 if the battery 15 is disconnected from the control circuit due to a collision. Specifically, it is impossible to change the activation of the airbag according to the demands of a user.
For avoiding the latter drawback, U.S. Pat. No. 5,187,465 to Stonerook et al. teach a dual airbag restraint system which includes an additional backup capacitor connected in series with a safing sensor corresponding to the safing sensor 3 in FIG. 3. This system, however, also has the drawback in that circuit components are increased.
U.S. Pat. No. 5,204,547 to Schumacher et al. teach the use of a single backup capacitor and a single safing sensor. All squibs are simultaneously activated in the event of a disconnection of the system from a vehicle's battery caused by an accident. This system, however, also has the drawback in that it is impossible to switch an airbag operation mode in accordance with user demands.